Travis CI
Continuous Integration service (self-check code on each git push) has free private builds for educational users, and free public builds for all.

Free continuous integration

These CI services are free for public repositories.
Some have usage limits (e.g. minutes/month).
Even for unlimited services like Travis CI, I cancel build jobs if I make git push in quick succession to save their resources from unneeded builds I know have errors.
A git commit including [ci skip] will not CI build, that seems to be a de facto standard.